This is not normal behavior. Usually the caterpillars are deep in the moss and undergrowth—away from birds and other predators. Once at the end of a branch, the caterpillar explodes when the baculovirus bursts out of its skin, looking to infect caterpillars below.

"It's like a zombie horror film," Chris Miller at the Lancashire Wildlife Trust, the wildlife charity that oversees the Winmarleigh Moss Nature Reserve, said in a statement.

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The infected caterpillars climb because the baculovirus affects how they respond to light. Once in a higher-than-usual location, they die, leaving their skin behind.

The zombie takeover, however, isn't a new phenomenon. It's been recorded for years, but this appears to be the latest "outbreak."